[5.04] Wave Supported clumps in Giant Molecular Clouds

We present further equilibrium models of molecular clumps
that are supported by Alfvén waves damped by the linear
process of ion-neutral friction. We used a WKB approximation
to treat the inward propagation of externally generated
waves and adopted a realistic ionization structure
influenced by dissociation and ionization due to photons of
external origin. The improved ionization structure results
in an ionization fraction in the envelopes of the model
clumps which is ~100 times greater than in other
studies, in which ionization structure that is more
appropriate for dark regions was assumed; thus, the waves
can penetrate more deeply. In cases where significant
internal dissipation occurred, the model clumps were found
to contain central condensations surrounded by flat,
extended, low density envelopes. The structures, and in
particular the central density, of model clumps, while being
fairly insensitive to the outer boundary density, depend
sensitively on the assumed depletions of sulphur and metals
as well as the frequency of the Alfvén wave and the
strength of the large-scale magnetic field. The model clumps
have central densities of ~104 cm-3, a
large-scale field of ~100 \muG, and a velocity
amplitude of a few km s-1 at a column depth
corresponding to 2 magnitudes of visual extinction.

For models which satisfy observational constraints on the
velocity amplitude of the waves, it is found that only a
narrow band of frequencies can contribute wholly to the
support of the clump. Specifically, we found that the ratio
of the strong coupling frequency limit to 2 \pi over the
wave crossing time is less than ~10. At higher
frequencies, the Alfvén waves are overdamped; at lower
frequencies, the clumps are smaller than the wavelength. The
resulting frequency range is far smaller than that normally
required for the establishment of an inertial power range
spectrum.

We will also discuss some observational consequences of our
clump models.

This work was supported by UK PPARC.

The author(s) of this abstract have provided an email address
for comments about the abstract:
robc@ast.leeds.ac.uk